ANCIENT CLAYS SUPPORT CONTEMPORARY BIOGEOCHEMICAL ACTIVITY IN THE CRITICAL ZONE

Late Cretaceous clays exposed at sites located on the north shore of Long Island, New York, USA were sampled to explore questions about how contemporary factors and processes interact with ancient geological materials. Chemically and biologically catalyzed weathering processes have produced multi-colored clays belonging to the kaolin group with inclusions of hematite, limonite, and pyrite nodules. We sampled exposed clays at three sites to address three questions: 1) Do these exposed clays support significant amounts of microbial biomass and activity, i.e, are they alive? 2) Do these clays support significant amounts of nitrogen (N) cycle activity? 3) Are these clays a potential source of N pollution in the contemporary landscape? Samples were analyzed for total carbon (C) and N content, microbial biomass C and N content, microbial respiration, potential net N mineralization and nitrification, nitrate (NO₃^-) and ammonium (NH₄⁺) content, and denitrification potential. Results strongly support the idea that ancient geologic materials play a role in contemporary C and N cycling in the Critical Zone. Respiration was detectable in all samples and was strongly correlated to organic matter, indicating a living microbial community on the clays. There was evidence of an active N cycle. Higher levels of denitrification potential compared to both potential net nitrification and potential net N mineralization indicate that these clays act more as a sink rather than as a source of N pollution in the landscape.

Major questions in Critical Zone science center on the ability of ancient materials to support biogeochemical processes related to the cycling of C and N that underlie plant and microbial activity, which underlies environmental and ecosystem “services” of interest to society. This investigation shed light on the amount and type of microbial activity that occurs in geological microhabitats in the Critical Zone at a coastal exposure of temperate northern latitude and allowed us to evaluate their potential local and regional impact. Our results advance the emerging science of the geological nitrogen cycle and clearly show that ancient geological materials are contributing to contemporary biogeochemical processes in the Critical Zone of this dynamic, densely populated, and environmentally sensitive region.